Various types of mechanical fasteners are known for connecting parts together. Nut and bolt combinations are typically used when high strength and/or disassembly is required. Under certain circumstances, however, even high-strength nut and bolt combinations can fail. Such failure can occur, for example, because of material defects, corrosion, fatigue, stress, etc.
In a typical joint having one or more bolts, each bolt provides a single load path at its location. Consequently, bolt failure at one location can result in a total loss of strength at that particular location, which in turn can cause undesirable load redistribution. To avoid single-point failures, dual load path fastener systems can be used in those applications where structural redundancy is required.
FIG. 1 is a cross-sectional side view of one type of dual load path fastener system 100 (“fastener system 100”) configured in accordance with the prior art. The fastener system 100 includes an outer bolt 110 extending through a first part 101 and a second part 102. The outer bolt 110 threadably engages a nut 127. Tightening the nut 127 puts the outer bolt 110 in tension and clamps the first part 101 to the second part 102.
The fastener system 100 further includes an inner bolt 120 extending through a hole 121 in the outer bolt 110. The inner bolt 120 engages internal threads 116 in a first head portion 112 of the outer bolt 110. Tightening the inner bolt 120 puts it in tension and compresses a flange of a cap 118 between a tip portion 113 of the outer bolt 110 and a second head portion 122 of the inner bolt 120.
If the outer bolt 110 fails in shear, the inner bolt 120 will carry the load, thereby providing a redundant load path or “fail-safe” feature in shear. If the outer bolt 110 or the nut 127 fails in tension, however, the inner bolt 120 may not carry the load. Thus, the fastener system 100 may be “zero failure tolerant” in these failure modes. For example, if the nut 127 fails, this will—at the very least—result in a loss of preload at the bolt location. Similarly, if the first head portion 112 of the outer bolt 110 fails, this will also result in a loss of preload—if not a total loss of tension capability—at this location. Loss of preload can have a number of negative consequences, including load redistribution to adjacent fasteners. This can overload the adjacent fasteners and/or the mating parts, ultimately leading to part or system failure.